This application is based on application No. 2000-80048 filed in Japan, the content of which is hereby incorporated by reference.
The present invention relates to a control method for an actuator including a shape-memory alloy and a control device for the actuator.
The demand for a small-sized position control mechanism, such as a hand shake correcting mechanism for a lens shutter camera, for example, has increased in recent years. An actuator that can meet this demand may comprise an actuator in which a shape-memory alloy (SMA) and a spring are used.
In an actuator using a shape-memory alloy, a shape-memory alloy that recovers to prescribed dimensions is used, and its temperature is controlled through the control of current that is supplied to the shape-memory alloy. Consequently, the shape-memory alloy is returned to the memory dimensions where necessary.
The temperature (point As) at which the shape-memory alloy begins to change its shape in order to return to the memory dimensions is determined based on the mixture ratio of the nickel and titanium that comprise the shape-memory alloy. The point As is set to a temperature slightly higher than the ambient temperature in order to avoid the situation in which the shape-memory alloy begins to change its shape when it is not intended to do so. For example, in the case of a camera, which is regularly used in everyday life, the point As is preferably 70xc2x0 C. to 80xc2x0 C.
However, if the point As is set to be higher than the ambient temperature, the problem arises that, when it is in fact desired that a change occur in the shape of the shape-memory alloy, a time lag occurs between the time at which application of voltage is begun and the time at which the shape-memory alloy actually reaches the point As, such that precise position control cannot be obtained. On the other hand, if the point As is set at a lower temperature in order to reduce the time lag, there is a possibility that the shape-memory alloy will begin to change its shape when it is not intended to.
An object of the present intention is to provide a position control device by which the unintended shape change of shape-memory alloy is prevented through the setting of the point As set at a high temperature, and using which very little time lag occurs when it is actually desired to change the shape of the shape-memory alloy.
The present invention was created in order to effectively resolve the above problems and provides a control device having the following features.
In the control device of the present invention, the actuator comprises a xe2x80x98shape-memory alloy that recovers to prescribed memory dimensionsxe2x80x99 and xe2x80x98a force application means that applies an external force to the shape-memory alloy and changes its dimensionsxe2x80x99. The position of a driven member in a first direction in which the shape-memory alloy returns to its memory dimensions, and in a second direction in which the force application means changes the dimensions of the shape-memory alloy, is controlled by the actuator. The control device of the present invention has a pre-heating means that, prior to the control of the driven member by the actuator, pre-heats the shape-memory alloy to a temperature at which the shape-memory alloy begins to change its shape or to a slightly lower temperature relative thereto.
In the control device of the present invention having the above construction, because the shape-memory alloy comprising the actuator is pre-heated to a temperature just short of the temperature (the point As) at which it starts to change its shape, when the applied voltage is increased in order to actually induce shape change, the shape-memory alloy begins to change its shape with very little time lag. Therefore, the degree of precision in position control increases, and it is also not necessary to set a low point As. The pre-heating means may heat the shape-memory alloy to a temperature higher than the above temperature in the initial stage of pre-heating. Using this construction, the time period needed for the shape-memory alloy to reach the desired pre-heated temperature may be reduced.
In the above control device, it is further preferred that a stopper that limits to a prescribed value the maximum amount of shape change of the shape-memory alloy carried out by the force application means. Using this construction, the initial position of the driven member before the shape-memory alloy begins to change its shape toward the memory dimensions due to heating may be accurately maintained at a constant position.
The above control device is suitable for position control of the shutter of a camera, and in such a case, it is preferred that the pre-heating means begin to supply current to the shape-memory alloy when the switch to begin various photo-taking preparation processes is turned ON.
The pre-heating means in the control device of the present invention may pre-heat the shape-memory alloy so that the driven member may be moved to a prescribed reference position before the control of the driven member by the actuator. This control device is suitable for position control of the correcting optical system of a camera having a hand shake correcting function, and in such a case, the center of the optical axis of the photo-taking optical system comprises the above reference position. It is preferred in this case as well that the pre-heating means begin to supply current to the shape-memory alloy when the switch to begin various photo-taking preparation processes is turned ON. The pre-heating means may heat the shape-memory alloy in the initial stage of pre-heating to a temperature higher than the temperature corresponding to the above reference position. Using this construction, the time period needed for the driven member to reach the reference position may be reduced.
Where the control device of the present invention is used in the shutter mechanism or hand shake correcting mechanism of a camera, the pre-heating means may be constructed such that it stops the supply of current when the photo-taking switch is not turned ON after the passage of a prescribed time period after the switch to begin photo-taking preparation processes is turned ON. Using this construction, where the shutter release button is pressed halfway down by accident and is maintained in that state for a long time, wasteful power consumption may be prevented.
In the control device of the present invention, the force application means may comprise a spring such as a bias spring, but the force application means itself may also comprise a shape-memory alloy. Where the force application means also comprises a shape-memory alloy, when current is being supplied to one of the shape-memory alloys (i.e., when pre-heating or full drive is being carried out), it is preferred that pre-heating be carried out with the other shape-memory alloy. Using this construction, highly precise position control with very little time lag may be attained regardless of whether the driven member is driven in the first or second direction.
These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate specific embodiments of the invention.